From the aspect of fish habitat formation, the investigation of step-pool system is important for the management of mountainous stream, since the step-pool system constructed by bed rock or many large rocks is indispensable for Yamame (Oncorhynchus masou masou) to form their habitat. The effect of large rocks in a pool, which are especially used for the temporal escape from a disturbance was analyzed based on the maximum numbers of fishes in a pool. The critical condition of pool size, related to the maximum numbers of fishes by two equations, was given as 52m^3 in volume of a pool, and the critical condition of rocks spaced in a pool, related to the maximum numbers of fishes by two equations too, was given as 0.08m^2/m^2 in horizontal area of large rocks occupied a pool. These result simply that the large rocks spaced in a pool play an important role for the formation of fish habitat when the large rocks occupy adequate area in a pool.